A high-pressure phase transition in chalcostibite, CuSbS (sub 2)

The crystal structure of chalcostibite alpha -CuSbS (sub 2) was investigated at pressures between 0 and 13.2 GPa by synchrotron single-crystal X-ray diffraction, compressing the sample under neon in a diamond anvil cell. The volume-pressure data, collected up to 10.7 GPa, were fitted to a third-order Birch-Murnaghan Equation-of-State, yielding V (sub 0) = 332.4(4) Aa (super 3) , K0 = 49(3) GPa, and K' = 6.9 (9). The compressibilities of the lattice parameters, up to 10.7 GPa, were calculated by fitting the data to a second-order Birch-Murnaghan EoS for a and b axes and the following values were found: K (sub 0) (sub a) = 49.0(6) GPa, a0 = 6.02(2) Aa, K0b = 76 (3) GPa, b (sub 0) = 3.80 (8) Aa. For the c axis a third-order Birch-Murnaghan EoS was used yielding to : K (sub 0) (sub c) = 33 (3) GPa, K' = 15(2), c (sub 0) = 14.51(1) Aa. At pressures higher than 11 GPa the evolution of the crystal lattice and geometrical parameters of the crystal structure indicate a phase transition from an orthorhombic Pnma to a monoclinic P21 structure. The single crystal survives the reversible phase transition despite the contraction of the unit cell of 2.2 vol% and the anisotropic change of the lattice parameters, which affects mainly the a lattice parameter. The lone electron pair stereo-chemical activity of Sb decreases with pressure as was evaluated by the reduction of the coordination-polyhedron eccentricity. The crystal structure of the high pressure beta -CuSbS (sub 2) was refined in the acentric P2 (sub 1) space group. Pressure induces shear planes in the crystal structure. The coordination number of Cu increases from four (tetrahedral) to five (square pyramidal). At the same time, the Sb coordination changes towards a distorted octahedral with a strong reduction of the stereochemical activity of Sb lone-electron pair and a movement of its centre closer to the Sb nucleus.